6 ED 15121
Modbus Protocol Information
Compatibility
The MicroTech III Chiller Unit Controller can be configured in an interoperable Modbus network. The controller must
have the corresponding Modbus Communication Module installed.
The MicroTech III Chiller Unit Controller conforms to the published Modbus standards. Refer to www.Modbus.org
for
more information.
Protocol Definitions
The Modbus protocol is a standardized Application Level (OSI Level 7) protocol used in interoperable Industrial Control
networks. Modbus provides the communication infrastructure necessary to integrate products manufactured by different
vendors and to integrate control services that are now independent.
It specifies how requests from the client are sent to a server and how servers reply. The client constructs a PDU (protocol
data unit) and sends it to a specific server or broadcasts it to all servers. The PDU contains a function code that defines
the action the client is requesting from the server(s). The PDU also includes a data field that further defines the action to
the server, for example, the location of the data to be read.
A normal reply from a server includes the same function code and a response data field. In the case of a read operation,
the response data field contains the requested data. In the case of a write operation, the response data field contains an
echo of the write data of the request command. If the server detects an error in the transmission, the reply to the client
includes and exception function code and the response data field contains an exception code.
Controllers can communicate on standard Modbus networks using one of two transmission modes: ASCII or RTU. Users
select the serial port communication parameters (baud rate, parity mode, etc), during configuration of the controller. The
mode and serial parameters must be the same for all devices on a Modbus network. Transmission mode determines how
information is packed into the message fields and decoded. In RTU mode, each byte contains two hexadecimal
characters, and in ASCII mode, each byte contains one ASCII character. The MicroTech III Chiller Unit Controller
uses the RTU mode only.
Valid Function Codes
The MicroTech III Chiller Unit Controller supports eight public function codes (see Table 1.) However, the MicroTech
III Chiller Unit Controller contains only Holding Registers (4xxxx).
Table 1. Valid Function Codes
Function Code Description Definition
01 (0x01) Read Coil Status Reads the On/Off status of discrete outputs.
02 (0x02) Read Input Status Reads the On/Off status of discrete inputs.
03 (0x03) Read Holding Registers Used to read 1 to approximately 125 contiguous input registers in a remote
device.
04 (0x04) Read Input Registers Reads the contents of input registers.
05 (0x05) Force Single Coil Force a single coil to on or off.
06 (0x06) Write Single Register Used to write a single holding register to a remote device.
15 (0x0F) Write Multiple Coils Forces each coil in a sequence of coils to either On or Off.
16 (0x10) Write Multiple Registers Used to write a block of 1 to approximately 120 contiguous registers in a remote
device.
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